Conventional units of this description, especially those having an essentially prismatic body , have plate-shaped components such as seats and backs or shelving and walls orthogonally interconnected by simple and not very reliable fasteners such as screws or nails, external attachments such as lugs or brackets, or laboriously machined formations such as mortises and tenons. The joints so formed not only tend to weaken the components but also mar the smooth exterior of the units.